The present invention relates to methods and apparatus for increasing facsimile call success rates. More particularly, the invention relates to facsimile call transmissions in the presence of long delays and channel errors such as those present in wireless access systems, e.g., terrestrial cellular, fixed wireless, and geostationary mobile satellite systems.
Conventional facsimile machines and compatible terminals are designed for communication over a public service telephone network. Standard protocols have been adopted for communication between calling and called facsimile terminals. Examples of such protocols are those defined by the International Telegraph and Telephone Consultative Committee (CCITT) under Recommendations T.3 and T.4, known respectively as the Group 2 and Group 3 facsimile protocols. Recommendation T.30 defines a protocol for Group 2 and 3 facsimile equipment for communication over a telephone network.
Previous techniques such as flag-stuffing require explicit knowledge of delays in the systems for it to be effective, since International Telecommunications Union standard ITU-T T.30 limits the duration of High Level Data Link Control (HDLC) frames, which includes HDLC flags. Knowledge of delay would assist the flag-stuffing technique to determine the time at which flag-stuffing should start so as not to violate the T.30 frame duration constraints.
An alternative means for communication between facsimile terminals has been proposed, in which each facsimile terminal is connected to a facsimile interface unit for communication via a public service telephone network to demodulate the signal that was modulated by the facsimile machine and transmit the demodulated data on digital satellite/cellular links, and modulate the data received over these links towards the end facsimile machines. This has the advantage of conserving bandwidth on bandwidth-limited satellite/cellular networks, since otherwise the modulated signal would have to be carried at the rate of 64 kbps. Thus, encoded data suitable for transmission over a digital network is provided, for example a digital satellite link or a cellular telephone system. However, the introduction of the Facsimile Interface Units (FIU) adds processing delay to the long propagation delays in satellite/ cellular networks that might further decrease call success rates. One example where the processing delay becomes a significant source is the facsimile demod-remod unit associated with Digital Circuit Multiplication Equipments (DCME), where processing/buffering delay can be as high as 300 ms. This increased delay may cause failures in communication between the calling and called facsimile terminals. A standard for overcoming this increased delay is not provided under the Group 3 fax protocol.
The document WO 92/02100 discloses a facsimile interface unit which automatically sends a xe2x80x9ccommand repeatxe2x80x9d signal to a facsimile terminal on receipt of a command therefrom, in order to allow more time for a response signal to be received.
The INMARSAT-B (TM) System Definition Manual, Issue 2, dated September 1989, proposes programming a facsimile interface unit to send a sequence of flags to a facsimile terminal if no response signal is detected within a predetermined period of receiving a command therefrom so that the time limits for response set out in Recommendation T.30 are not exceeded at the facsimile terminal.
Another scheme that is used in the Global System for Mobile Communications (GSM) non-transparent facsimile service (GSM 03.46) is blind blocking of retransmitted commands. Here the facsimile adapter blocks re-transmitted commands from reaching the remote end until a response is received from the remote end. This response is then forwarded to the command sending entity which thinks that it is a response to the most recently retransmitted command although it is a delayed response to the first transmitted command. This scheme has the potential danger of timing out at the postmessage phase of the facsimile call if too many retransmissions occurred during image transfer phase of facsimile call. WO 95/22224 provides a solution to the long delay problem by using the flag-stuffing, where the HDLC flags are autonomously generated by Intelligent Facsimile Relay (IFR) equivalents. The facsimile interface unit disclosed in WO 95/22224 detects a transmitted signal from a transmitting facsimile apparatus and detects whether a response signal to said transmitted signal is received from another facsimile apparatus within a predetermined period. If no response signal is detected after a predetermined period, a command-repeat (CRP) is transmitted to the command-sending entity. This forces the command sending entity to repeat the previously transmitted command until a response is received from the remote end. While this takes care of the time-out problem of T.30, the problem of collision at the 2-wire link is not solved. This is especially true when there is a long delay in the Public Switch Telephone Network (PSTN) leg of the connection and the delay is unknown. A consequence of this is that the CRP is received in error by the command sending entity.
Collision avoidance techniques that do not need a collision detector have been well published in GSM 03.46 and ITU-T X.38. Here the Fax Adapter (FA in GSM 03.46) or FPAD of ITU-T X.38, blindly blocks or xe2x80x9cignoresxe2x80x9d a retransmitted command which will prevent the retransmitted command to go and collide with a response to the command at the far end. However, collisions may still occur at the near end if the time of transmission of the retransmitted command coincides with arrival of response. To solve this problem, GSM 03.46 uses a supervision timer which essentially is an estimate of a safe period in which a response can be relayed to the command sending entity without colliding with a retransmitted command. The timer is started upon arrival of a command at the FA. If the response is received by the FA after the supervision timer has expired, then the response is buffered until the arrival of a retransmitted command and then relayed to the command sending entity. Here collision is avoided on the PSTN side which has a half-duplex modem based on supervision timer.
While blind blocking, flag-stuffing techniques and the like may be used for transparent facsimile services that do not use retransmission of GSM air interface, it poses the danger of loss of the very first transmitted command on the air interface, making the scheme too sensitive to channel impairments. Furthermore, many facsimile machines ignore the first DIS command.
This invention provides a technique that will permit reliable facsimile transmission in presence of long delays. The main advantage of this technique is that it does not require specific knowledge of the delays in the system in order to avoid signal collision and to keep the connection alive. An important assumption here is that the IFR does not have the capability to detect collision, which is typical in many practical implementations where the relay uses off-the-shelf modem chip-sets that operate in half-duplex manner.
Essentially the methods and apparatus disclosed in the described embodiments provide monitoring and/or manipulation through the use of entities, referred to as Intelligent Facsimile Relays (IFRs) that are physically located between the two end facsimile machines communicating over a long delay and possibly impaired link. The IFRs constantly monitor (and if necessary manipulate) the ITU-T T.30 protocols in both directions and intelligently decides to transmit, relay, buffer, or discard individual messages in the T.30 protocol. The primary intent is to avoid signal collisions on 2-wire interfaces (or 4-wire interfaces with half-duplex implementations) and simultaneously prevent disconnects due to repeated ITU-T T.30 time-outs. A similar problem also exists on links that may be 4-wire or full-duplex channel in nature, but the link is controlled via an entity whose operation is half-duplex in nature. In both cases, the effect is that the signal is lost. The primary goal of the proposed solution is to complete the pre-message (Phase B as defined in ITU-T T.30) and post-message (Phase D as defined in ITU-T T.30) handshaking of the T.30 successfully, by having the IFR monitor, store, and regenerate the T.30 handshake messages and responses, without the need for collision detection.
In addition to monitoring, whenever it is necessary to manipulate T.30 messages (such as manipulation of user rates in Digital Identification Signal (DIS), intentionally disabling Error Correction Mode (ECM) mode of operation, etc.), the IFRs perform a forced corruption if the IFRs themselves receive a T.30 message in error. Furthermore, the IFR appropriately generates and transmits standard non-user specific T.30 messages in place of the messages received from the end facsimile machines, thereby preventing the possibility of blindly relaying a possibly corrupted message to the other end. This provides additional robustness to the T.30 protocol handling.
An advantage of the disclosed technique is that it does not require specific knowledge of the delays in the system in order to avoid signal collision and to keep the connection alive, nor does it need the mechanism to detect signal collision. Other described approaches such as the collision avoidance technique, centered around a collision detection mechanism, is not available in many existing fax relays having fax modem chip-sets that operate in half-duplex mode, thereby preventing xe2x80x9cdetectionxe2x80x9d of collision. Hence a reliable mechanism is necessary to cater to such implementations.
This invention thus builds upon the collision avoidance techniques described in GSM 03.46 and ITU-T X.38; however instead of blind blocking or ignoring of retransmitted, selective blocking of retransmitted commands is proposed, that makes the protocol less sensitive to channel errors that are typical in wireless channels. In addition, the invention improves upon techniques described in GSM 03.46 and ITU-T X.38 for preventing time-out between low speed message ITU-T T.30 signal and high speed ITU-T T.4 page data.
Although, collision avoidance techniques that do not need a collision detector may ignore retransmitted commands to prevent a retransmitted command from colliding with a response from the far end, collisions nonetheless occur at the near end if the time of transmission of the retransmitted command coincides with arrival of response. To solve this problem, a supervision timer may be employed to estimate of a safe period in which a response can be relayed to the command sending entity without colliding with a retransmitted command. Thus potential collisions are avoided on the PSTN side which has a half-duplex modem based on the supervision timer.
However, if it is known that the wireless channel (which, in general, is a full-duplex channel) is controlled by an entity that operates in a half-duplex manner, then the technique of using supervision timer of 03.46 can be applied towards wireless channel to avoid simultaneous transmission and reception on wireless link. One such wireless channel that ignores data arriving on the wireless channel when it is receiving data from its near-end fax machine is the GSM Transparent fax channel. The extension of GSM 03.46 collision avoidance technique to the satellite side is applicable for such channels. Where the delays in the systems are long such that a response is always provided to a command-sending entity after it has retransmitted a command and if second retransmitted command is always blocked, then the protocol becomes extremely sensitive to errors on the channel. If the first command or first response is lost, then this would lead to a call disconnect. The invention provides techniques to improve the robustness of the protocol when collision avoidance techniques described in GSM 03.46 and ITU-T X.38 are used in wireless environments. An embodiment is described in terms of the functionality of the IFR. Although most of the description is centered around the IFR located in the base station (referred to as Network_IFR), most of the techniques are equally applicable to the IFR located at subscriber side (referred to as Subscriber_IFR). Applicability does not imply that the solutions have to be implemented on both sides of the connection.
Briefly summarized, the present invention relates to a facsimile transmission relay apparatus for improving call success rates in long delay environments. The apparatus disclosed in the embodiments relate to an intelligent facsimile relay having a first bi-directional telecommunication interface for receiving a facsimile signal from a transmitting facsimile device via a first network, such as the PSTN. The relay apparatus is provided with a buffer memory for storing the facsimile signals from the transmitting facsimile device, and a second bi-directional telecommunication interface facilitates the retransmission of the facsimile signal via a second network to a receiving facsimile device, e.g., via a wireless telecommunications network. An information processor is coupled to the second interface monitoring messages from the receiving facsimile device and for generating messages to the receiving facsimile device in order to maintain communications from the transmitting facsimile device via the first and second networks, without the need for signal collision detection but rather providing selective blocking of retransmitted commands for less sensitivity to the channel errors present in wireless applications.
It will be understood that both the foregoing general description and the following detailed description are exemplary and intended to provide further explanation of the invention as claimed. The accompanying drawings provide an understanding of the invention as described in the preferred embodiments to illustrate the invention and to serve to explain the principles of the invention.